First thing that comes to mind: the hoverboard. That would just be really neat/funny. I guess an even greater anti-gravity machine would be like what holds up the cars in The Fifth Element and other movies like Star Trek: Into Darkness though. No more traffic, no more car accidents, more efficient air travel, etc. But the ultimate invention would be the atomizer from Elysium. You could cure any sickness. You could live forever. (Well, living forever probably isn't actually a good thing, but it's fun to think about under a certain frame.) A corollary is the replicator from Star Trek. It can arrange matter into any form like the atomizer (I guess the atomizer more rearranges matter though, and the replicator isn't used to cure people). Another corollary is the transporter in Star Trek, because it can actually be used to manifest an arbitrary object/person by downloading from a saved state. You should be able to modify the saved state to manifest anything you want to. The Holodeck from Star Trek is pretty cool too, though.. come to think of it, I think I'd rather that than the atomizer. I'd rather have a lot of fun (for example, snowboarding or rally racing without any danger) than prolong my life. But I guess, what's better for the world at large is the atomizer because it could prevent a lot of pain, suffering, death and loss.